This invention relates to an improved process for separating crystallizable C.sub.3 polyol monoacylates from a feed mixture thereof with related polyol polyacylates.
The C.sub.3 polyol monoacylates of concern herein are the monoacylates (monoesters) of glycerine and propylene glycol, i.e. monoglycerides and propylene glycol monoesters of C.sub.2-26 fat-forming acids. Typical such fat-forming acids include the acids: lauric, myristic, palmitic, oleic, stearic, butyric, linoleic, behenic, elaidic, and like fatty acids. Preferably such mono-acylates are edible, advantageously with C.sub.8-26 acyl groups, and most commonly and preferably with C.sub.12-22 acyl groups. While the crystallizable mono-acylates most readily and preferably handled in this process are those having reasonably high capillary melting points (eg. at least 100.degree. F. or higher), those lower in melting point also can be separated effectively by the process.
Crystallizable C.sub.3 polyol monoacylates form crystals separable from the parent mixture by cooling such mixture, eg. to a temperature below about 50.degree. C. The temperature at which such monoacylates crystallize can be as low as about -40.degree. C. or even lower. It may be advantageous on occasion to employ superatmospheric or subatmospheric pressure during fractional crystallization for special effects, but preferably, for efficiency and economy, atmospheric pressure is used for the fractional crystallization. Fractional crystallization of feed mixture for this process generally is carried out by dissolving the mixture in a solvent, cooling the solution until a crystalline fraction is formed, and separating said crystalline fraction from remaining solution. Often such process is termed "solvent fractionation".
Monoesters of glycerine and propylene glycol (monoglycerides and monoglycolates) have many uses such as emulsifiers, particularly in foods, cosmetics, etc.
Heretofore, various processes have been suggested for separating a fatty mixture into various fractions enriched in one or more of mixture components and depleted in others by fractional crystallization of such mixture. One such process (Canadian Pat. No. 751,920) shows the separation of an .alpha.-monoglyceride from a mixture of monoglycerides with di- and tri-glycerides by dissolving such mixture in 2-nitropropane solvent, cooling the solution to form a crystalline fraction, filtering off the crystals (rich in monoglycerides) from the remaining mother liquor, washing such crystals with additional cold 2-nitropropane solvent, and removing residual solvent from the crystals by vacuum distillation or by steam distillation. It should be noted here that distillations tend to degrade concentrated monoglycerides at even modestly elevated distillating temperatures, and steam distillation tends to degrade such monoglycerides even at fairly low distilling temperatures, thus substantially detracting from the concentration effected in the prior fractional crystallization operation.
Another prior proposal (U.S. Pat. No. 2,608,564) suggests the separation of polyhydric alcohol partial ester mixtures into its various components based on degree of saturation of such components by dissolving the mixture in a solvent of alcohol plus monocarboxylic acid, cooling the mixture to form a crystalline fraction, separating the crystals from the remaining mixture, washing the crystals with additional cold solvent, and drying the washed crystals in their solid state.
Still another prior proposal (U.S. Pat. No. 2,450,235) suggests the separation of fatty oil substances into components of such mixture based on degree of saturation of such components by dissolving the mixture in a solvent such as an aliphatic ketone, cooling the mixture to form a crystalline fraction, separating the crystalline fraction from remaining mixture by vacuum filtration, washing the crystalline fraction with additional cold solvent, and passing cold inert gas through the solid crystalline filter cake under vacuum to remove residual wash solvent and to maintain the crystalline fraction in cool solid state.
Yet another prior proposal (U.S. Pat. No. 2,934,547) suggests the separation of glyceride oils into component fractions based on degree of saturation of such fractions by dissolving the oil in a polar solvent, melting the crystalline cake at about 121.degree., removing solvent therefrom by flash evaporation at 138.degree.-160.degree., then passing inert gas through the concentrated melt to remove residual amounts of solvent. Still other proposals (U.S. Pat. Nos. 2,684,377 and 2,684,378) separate triglyceride oils such as peanut and cottonseed oils into component fractions based on degree of saturation of such fractions by dissolving the oil in a solvent composed of polar solvent and a normally liquid hydrocarbon (such as hexane), chilling the mixture, removing the crystalline fraction formed from remaining mixture by centrifuging, and heating the removed crystalline fraction to about 100.degree. under reduced pressure of 10 millimeters of mercury with a stream of nitrogen gas being passed therethrough to remove residual amounts of solvent.
Advantages of this invention over prior proposals include the ability to produce a concentrated C.sub.3 polyol monoacylate fraction economically and in good purity. Virtually all of the solvent is removed from this product without causing substantial molecular rearrangement or other degradation of the concentrated monoacylate product.
The instant invention is based on several factors not readily apparent from or recognized by the abundant art on fractional crystallization of fatty substances in general, and C.sub.3 polyol monoacylates in particular. One factor is that the presence of steam, water, or other hydroxylated solvent in such crystal crop tends to degrade and cause loss of the monoacylate in the crop even at temperatures of 100.degree.-150.degree., forming free polyol, free fatty acid, polyol diacylates, and polyol triacylates. Correlative with this is the avoidance of otherwise popular steam stripping. Said Canadian patent suggests steam stripping; thus clearly it is unconcerned with the degradation this can bring. The maximum stripping temperature of 150.degree. also is quite critical as a further factor for prevention of the same kind of degradation, even in the absence of hydroxylated solvent.
Thus, the instant process can be characterized in part as providing effective stripping conditions for removal of such free polyol while substantially precluding degradation of the concentrated monoacylate product, a consequence not heretofore taught or probably even recognized in the art of monoglyceride partial crystallization (which even proposed clearly deleterious steam distillation of such product).